The present invention relates to multi-phase AC induction motor control, in general, and more particularly to multi-mode control system for an induction motor capable of operating from idle to run speed and conversely to a stop.
Solid-state AC motor starter circuits are generally known. See for instance "Design and Application of a Solid-State AC Motor Starter" by John Mungenast in IEEE Transactions, Vol. IA-12, No. 1, 1976, pp. 39-42. Starting requirements of an induction motor have been reviewed in "Starting Requirements of Pipeline Motor" by Howard E. Barr, in IEEE Transactions, Vol. IA-15, No. 6, 1979, pp. 625-629. A most commonly used mode of starting an induction motor is by reduced-voltage. See for instance "Reduced-Voltage Starting of Squirrel-Cage Induction Motors" by Frank M. Bruce, R. J. Graefe, Arthur Lutz and M. D. Panlener, in IEEE Transactions, Vol. IA-20, No. 1, 1982, pp. 46-55.
Problems and solutions involving motor starting have been considered in U.S. Pat. Nos. 4,384,243 and 4,482,852 of Nicholas G. Muckovac. See also "Service to Induction Motor" by Archie W. Cain, in IEEE Transactions Vol. IGA-7, No. 3, 1971, pp. 359-366; and see U.S. Pat. No. 4,482,853 of M. R. Bhavaar.
The present invention involves control of an AC motor through SCR's for soft start as well as for a stop. Much of the former practice with this type of AC motor starter or motor stopper relied on analog gate pulse generators. Starting or stopping of an induction motor may go through a number of critical stages, or situations, that can occur within a very short time span. With an SCR gate pulse generator, as commonly used with a 3-phase motor, accurate timing of the SCR gate pulses is necessary, but difficult to achieve considering the many and fast response steps involved in the starting process. Also, it is not always possible to obtain zero to 100% range of control at all power factors. The motor starting or stopping procedure may involve a series of critical events between idle and normal operative speed of the motor. Among these, there is an increased, or a reduced voltage, applied to the motor during the critical period, obtained by phasing the firing angle of the SCR's far enough, which action may have to be countermanded. The starting or stopping procedure may have to be interrupted, or modified to respond to critical events encountered in the process. These events may be detected as a result of sensing the load, the slip, or the current of the motor. The object of such detections is to ensure a smooth and safe start up or stoppage, to protect the motor against an abnormal situation, such as current surges, excessive slip or torque, and there may be a need for reacting immediately to trip, or compensate. The invention applies to a situation where the only controllable unit in each such situation is the gate pulse generation system which normally establishes a predetermined motor operation, but also which commands ramping up the motor in speed for a soft start or ramping down to stop the motor. The gate pulse generator will be used to interrupt a soft start, to trip the operation and eventually to reinitiate the starting procedure. The problem lies here in providing an interface between the various channels, which are affected to the respective critical event, and the gate pulse generator of the induction motor, so that potential critical control and/or command signals which would prevail at a given instant, or in a given situation, may be individually applied and independently used for controlling the gate pulse generator of the induction motor, whereby the latter can respond adequately and fast enough during the starting procedure, or in the stopping process, upon the occurrence of any such particular critical event.
The present invention combines and integrates the functions of a plurality of control channels which are associated with the respective such potential independent and critical events in such a way that (1) any channel may override a running channel upon the occurrence of a correlated and prevailing event; and (2) the gate pulse generator can respond adequately and quickly to such intervening channel to control the SCR switches so as to establish immediately and appropriately an AC voltage for the motor as required by the situation.